Introduction

The Omniverse Theory.
The model of the entire existence.
Also known as the Theory of Literally Everything.

Not to be confused with a Theory of Everything, which is concerned with fully explaining the rules governing the universe.1
It should not be confused with mutltiverse hypotheses, either. These, in turn, attempt to describe what is beyond the universe. A multiverse hypothesis might be seen as complementary to the Theory of Everything - together, they could explain both the inside and the outside of the universe.

The Omniverse Theory has no such ambitions. While it provides an approach to defining existence and explores its implications, it does not seek to describe reality nor explain natural phenomena. It matches what is known about the real world and other imaginable worlds alike, and gives an idea about their possible origins - but these origins are not the primary concern. More focus is given to exploring the variety of cosmic systems and possible interactons among universes. From this perspective, this theory can be seen more as a philosophical pursuit for its own sake, rather than an answer to a problem.

A more pragmatic way to look at the Omniverse Theory is a toolkit for worldbuilding and storytelling - a meta-setting, from which other settings emerge, and recurring concepts and patterns. There are many levels on which the Omniverse Theory may apply - from the most fundamental and abstract (dealing with all existence as a whole), through relationships among particular universes, to very specific (e.g. how the difficulty and flexibility of shapeshifting depends on world mechanics). Of course, this theory isn't required to write engaging fiction or build wondrous worlds, but it may provide a new perspective or additional inspiration for one's works.

As a general "Don't try this at home!" disclaimer, I'd warn against overthinking the Omniverse Theory, especially in the context of reality. It's nice to occasionally consider different possibilities, but in the end one needs to keep themself alive, care for people close to them, things like that. Please don't make a religion or pseudoscience out of it, either; it's not what it's meant for at all!

Why Omniverse?

The Omniverse Theory has many similarities to multiverse hypotheses, as it also deals with possibilities of other worlds. From that perspective, using the word "Omniverse" might come across as a forced distinction from other multiverse systems, or worse, as a desperate attempt to sound cooler or grander than "those other theories." However, I have specific reasons behind such a choice.

First reason, rather nitpicky: I'm not fond of the "multiverse" definition summary from Wikipedia (as of 24 July 2017):

The multiverse (or meta-universe) is the hypothetical set of possible universes, including the universe in which we live. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, and the physical laws and constants that describe them.

Listing the "space, time, matter, energy" along "physical laws and constants" puts all these features on the same level. It suggests the former four items are as essential as the other two. In the Omniverse Theory, this is not necessarily the case; universe lacking space or matter is perfectly feasible, unlike a universe without any laws whatsoever. Thus, this take on "multiverse" is not something I'd like the Omniverse Theory to be associated with.

Second reason, more relevant: the very word "multiverse" suggests "many universes" (multi-verse), not "all possible universes." "Omniverse" conveys the latter idea more clearly, especially to those who are unfamiliar with either term. I would rather use "multiverse" for an arbitrary group of universes, but it already has an established meaning. To avoid confusion, I will describe such a group as "polyverse" instead.

Which Theory?

More scientifically inclined people could call me out on a carefree use of term "theory." Granted, it is often misunderstood as little more than a speculation, whereas in the context of natural sciences it means a well-established and verifiable explanation of observable phenomena.2 From this perspective, "theory" seems way too flattering for the Omniverse Theory.

Then again, the Omniverse Theory doesn't attempt to explain the world as natural sciences do. It describes a system that would produce the known universe (among other things), but doesn't claim to be the system the universe originated from. In that regard, it's more akin to mathematical frameworks (like the dimension theory or the set theory) than to scientific theories (like the theory of relativity, the theory of evolution or the elusive Theory of Everything).
I can't guarantee that the Omniverse Theory will always adhere to formal rigor and precision, but I'll do my best to keep it coherent and specific.

Technically, based on the Omniverse Theory, an Omniverse Hypothesis can be formulated: "The Omniverse Theory does apply to reality." However, this hypothesis is too general to be verifiable or constructive, and thus quite useless for natural science. That's the main reason why the Omniverse Theory works better as a standalone system, mostly unconcerned with the real world.3

What's next

I plan to write and release short chapters describing the Omniverse Theory in more detail, once a week or two (I will aim for the former). I will start with the core concepts and principles, setting up the foundation, before getting to how universes vary among each other and their typical components. Stay tuned!

^ The universe understood as the cosmos - the system of laws, dimensions and elements that humans live in - rather than the whole existence.

^ That's why a stock phrase like "evolution is just a theory" can be quite jarring, especially to scientists who have yet to be disillusioned with humans' reasonability.